DISEASE
MANAGEMENT: CHEMICAL CONTROL

Introduction

Chemical disease control employs the use of chemicals that are either generally
toxic and used as disinfectants or fumigants or chemicals that target specific
kinds of pathogens, as in the case of fungicides, bactericides (or antibiotics)
and nematicides. No substances have yet been found that effectively
control plant viruses.

Ideally, a chemical control agent should be effective at concentrations
that will not harm the plant, have low risk to humans and animals, and have
minimal effect on the normal microflora on the plants and in the soil.
Also, there should be little chance of the pathogen quickly developing resistance
to it, and it should be suitable for long periods of storage in ambient
conditions. These chemical agents can be sold as dusts, concentrated
solutions, wettable powders, granules or emulsions.

FUNGICIDES

Fungicides are chemicals used in the control of fungal diseases.
They are often classified as either protectant or systemic. Protectant
fungicides are usually effective against a broad range of fungi and
protect the plant against infection on the surfaces of the plant to which
they are applied. Often, they require multiple applications during
the growing season to maintain coverage as new growth emerges and weathering
removes past coverage. Systemic
fungicides can be absorbed by the plant without harming it, and transported
to other tissues where they are toxic to fungi. These compounds can
control and eradicate established infections, but they are also vulnerable
to fungi developing resistance, as they generally only target one step in
a biosynthetic pathway to kill the fungus. To minimise the development
of resistance by chemical overuse, fungicides are classified into groups
based on their chemical activity. By alternating between the different
classes of fungicides, the fungal population has less opportunity to build
up resistance to one chemical. Link
to Fungal Biology - Fungicides.

ANTIBIOTICS

Relatively few antibiotics are routinely used to control plant diseases.
Antibiotics are chemical produced by micro-organisms, which destroy or injure
living organisms, in particular, bacteria. Streptomycin is effective
against a few fruit pathogens, such as blights and cankers, and cyclohexamine
can be used to control some fungal pathogens of crops, particularly powdery
mildews and rusts. Bacteria, as well as fungi, have the ability to
develop resistance to antibiotics, which is a major disadvantage of using
these compounds, and one of the reasons that they are not widely used. Link
to Fungal Biology - Fungicides.

NEMATICIDES

The use of nematicides is confined largely to high-return horticultural
crops, because they are expensive. Additionally, they are all highly
toxic, and alternative measures for controlling nematodes are being investigated.
Link
to Fungal Biology - Nematodes.

ISSUES RELATED TO CHEMICAL DISEASE MANAGEMENT

Pathogen resistance to chemical agents has been a problem since the 1970s.
Throughout the 1980s and 1990s, public concern has grown about the use of
agricultural chemicals in general. The main concerns are the risk
of poisoning humans or animals, contamination of livestock products, harm
to beneficial insects, and the contamination of food products, waterways
and soil. When properly used, fungicides should not cause problems
in any of these areas, but they do have the potential to alter the balance
of diseases on certain crops.

The main risk to humans is during the preparation or application of these
chemicals, when they can be inhaled, ingested or absorbed through the skin,
and upon consumption of plants or their products. To overcome these
risks, safety precautions must be followed when working with the chemicals,
and various regulations exist with regard to consumable plant products.
For example, some chemicals may not be applied within a certain time before
harvesting (the with-holding period), thus ensuring that chemical residues
have fallen to an acceptable level by the time they reach the market.
Maximum residue limits have been established to allow monitoring of the
chemical residue on plant products.

Environmental concerns focus mainly on protectant fungicides. For
example, copper and sulphur sprays have the potential to affect a broad
range of organisms if they are washed off the leaves and accumulate in the
soil or are washed into the waterways.

In response to public concern about these issues, many countries have initiated
programs to reduced pesticide use. They can involve education of farmers
on best practices in pesticide application, the use of lower doses of chemicals,
better understanding of the threshold level of infection that causes economic
loss and developing disease forecasting systems to allow optimal use of
chemicals.